And Ed Miliband was put on the spot over the last Labour government's spending.

Mr Miliband was also pressed on whether he would do a deal with the Scottish National Party and insisted there would be no Labour government if it meant relying on their support.

In a 90-minute Question Time special, the two leaders and Nick Clegg faced a studio audience separately in Leeds Town Hall, in a format devised after Mr Cameron refused to take part in a head-to-head encounter with his Labour rival.

Mr Cameron rejected suggestions he was planning to slash £8 billion from child benefit and child tax credit, but both Mr Miliband and Mr Clegg retorted that his comments did not amount to a guarantee that the benefits were safe.

Mr Miliband said he did not accept that the last Labour government overspent while in office and promised the party was "absolutely, deadly serious" about balancing the books in the next parliament.

And the Labour leader gave his strongest indication yet of his unwillingness to strike a deal of any kind with the SNP in the event of a hung Parliament, telling the audience: "If the price of a Labour government is a coalition or a deal with the SNP, it is not going to happen."

An instant poll of Question Time viewers conducted by ICM for The Guardian found that 44% thought Mr Cameron had "done best on the night", against 38% for Mr Miliband and 19% for Mr Clegg.

Speaking after the broadcast, Tory Leader of the Commons William Hague said that Mr Miliband had failed to rule out attempting to govern as a minority administration propped up by SNP votes.

"Ed Miliband ruled out coalitions and deals with the SNP, but what he means of course is that every day there would have to be a vote, a negotiation, and everybody held to ransom by the Scottish National Party," he said.

But Labour shadow cabinet member Caroline Flint insisted: "Ed Miliband made it absolutely clear - we are not into this posturing and bartering, there will not be a coalition, there will not be a deal with the SNP."

Mr Miliband said his message to Scottish voters was: "There's no easy route here to vote SNP and get a Labour government. If you want a Labour government you need to vote Labour."

And he insisted: "If we don't have a majority government it is not about saying 'Go into a darkened room with somebody and start lopping off bits of your manifesto'."

Mr Cameron was put on the spot over Tory plans for child benefit after Liberal Democrats revealed details of proposals discussed by the coalition for £8 billion cuts to the payments, including means-testing payments, removing them from 16-to-19-year-olds and limiting the benefit and child tax credit to two children per family.

Mr Cameron said: "I don't want to do that. This report that was out today is something I rejected at the time as Prime Minister and I reject again today."

Asked if this amounted to an "absolute guarantee" not to cut child benefit or child tax credit, the Conservative leader said: "Child tax credit we increased by £450. That's not going to fall. Child benefit, to me is one of the most important benefits there is ... It is the key part of families' budgets in this country. That's not what we need to change."

But Mr Miliband said that the Prime Minister had not definitively ruled out the cuts, telling an audience member: "He might have sounded like he answered your question but he didn't, he didn't really give you a guarantee.

"I'm going to give you that guarantee tonight, I'm not going to cut your tax credits, I'm not going to cut child benefit or means-test it ... I've got to say after Mr Cameron's answers tonight, tax credits and child benefit are on the ballot paper at this election because millions of families risk losing thousands of pounds."

Mr Clegg said the £12 billion cut was equivalent to scrapping the whole budget for disability living allowance or employment allowance.

He added: "Unless the Conservatives come clean - and I don't think David Cameron has done it this evening - where they are going to take the equivalent of £1,500 off eight million of the most vulnerable families? We can only assume that they are looking at the kind of plans which they floated some years ago in government."

Meanwhile, an audience member directly asked Mr Miliband: "Do you accept that when Labour was last in power it overspent?"

The Labour leader replied: "No, I don't ... There was a global financial crisis which caused the deficit to rise. President Obama isn't dealing with a high deficit because we built more schools and hospitals. He is dealing with a deficit because of the global financial crisis."

As he explained the financial difficulties the Coalition faced when it entered office in 2010, Mr Cameron took from his pocket the famous letter left by former Treasury minister Liam Byrne telling his successor "there's no money left".

He said he had been "carrying it with me everywhere" during the campaign, but Mr Miliband dismissed it as "his regular prop".

Mr Clegg reiterated his assertion that the Lib Dems would first try to seek a coalition deal with the largest party in terms of votes and seats after May 7.

But the Deputy Prime Minister indicated that if a deal cannot be struck then he would consider speaking to the second-placed party.

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